Generally, photodiodes have a cosine angular response, meaning that the peak response of the photodiode is achieved at a normal angle of incidence where light is impinging perpendicular to the surface. This response gradually decreases according to the cosine function to a zero output at 90°.
This cosine response is a drawback in some types of solar sensors. In some vehicles, a solar sensor is used to measure solar heating by sunlight. The sensor represents a sampling of the heating affect occurring on some object, such as a vehicle. However, the solar heating affect only follows the cosine response for objects that are flat. Thus, the use of photodiodes is sometimes limited to modeling the heating of flat objects.
However, many practical solar sensor applications, including especially those with a passenger compartment of a vehicle, are helped by sensors whose response corresponds to such complex three-dimensional shapes.
One of the design goals of automotive solar sensors is to respond to sunlight in a fashion that is consistent with the heating affects on the passenger compartment. In general terms, the desired overhead response is about 50% of the peak response, due to the shading effects of the roof. The peak response typically occurs at about 50° from overhead. The response at the horizon is generally desired to be about 50 to 70% of the peak response, due to the relatively large area of glass exposed in that angular region.
Some automotive solar sensors use a domed diffuser to provide increased response when the sun is near the horizon. The thicker top section reduces the overhead response inherent in the photodiode's cosine-related angular response. One difficulty with this approach is the significant reduction in overall signal current due to the loss of light through the diffuser material. In some solar sensors, the use of a diffuser provides lower signal output for a given size diode, requires a larger diode to achieve a given signal output level, may require additional signal amplification for proper signal processing, and may be characterized with a decreased signal to noise ratio due to the attenuated signal.
What is needed are apparatus and methods which overcome the problems in other solar sensors. The present invention does this in a novel and unobvious manner.